1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to an active-matrix-addressed liquid crystal display using a thin film transistor (TFT).
2. Description of the Prior Art
An up-to-date active-matrix-addressed liquid crystal display has been greatly improved, and actively applied to a processor system such as a laptop personal computer in view of aspects such as high quality and pixel density of an image.
The active-matrix-addressed liquid crystal display includes an active-matrix substrate and an opposing electrode substrate. There are formed an array of drive elements such as thin film transistors (TFTs) on the active-matrix substrate, and opposing electrodes over the display area on the opposing electrode substrate, respectively.
FIG. 13 is a plan view of a conventional active-matrix substrate, and FIG. 14 is a sectional view taken along the line A--A of FIG. 13. In FIGS. 13 and 14, reference numeral 1 denotes a glass substrate; 3 a data line conductor; 4 a gate bus strip; 5 a thin film transistor; 6 a pixel electrode; 7 a passivation layer; and 10 an alignment film.
FIG. 15 is a plan view of a conventional opposing electrode substrate, and FIG. 16 is a opposing electrode substrate, and FIG. 16 is a sectional view taken along the line A--A of FIG. 15. In FIG. 15, the figures or contours defined by generally rectanglar shapes 8a are representative of openings of black mask 8. In FIG. 16, reference numeral 2 denotes a glass substrate. There are formed on one of the major surfaces of the glass substrate 2 the black mask 8 comprising a thin film of a metal such as chromium (Cr), and over the display area an opposing electrode 9 comprising ITO (Indium Tin Oxide) and alignment film 12, respectively.
The active-matrix-addressed liquid crystal display comprises an assembly of those two substrates in their combination in which a liquid crystal is contained in a space 14 formed between the two substrates.
FIG. 17 is a plan view of a conventional active-matrix-addressed liquid crystal display, and FIG. 18 is a sectional view taken along the line A--A of FIG. 17. As can be seen from FIGS. 17 and 18, wiring areas of the data line conductors 3 and the gate bus strips 4, and areas of the thin film transistors 5 on the active-matrix substrate 1 are optically shaded by the black masks 8 on the opposing electrode substrate 2.
There are two objects of providing the black masks 8, one of which is to shade the thin film transistor channel area so as to suppress a photoelectric effect due to incident light from the side of the opposing electrode substrate. The other object is, as described in references: (1) A. Lien, "Two-Dimensional Simulation of the Lateral Field Effect of a 90.degree. TN LCD Cell" I. D. R. C. Eurodisplay '90 Digest, pp. 248-251; (2) T. Onozawa, "Influences on Director Alignment of the Lateral Bus-Line Field in an Active-Matrix-Addressed Liquid Crystal Display" Japanese Journal of Applied Physics, Vol. 29, No. 10 (1990) pp. L1853-L1855; (3) E. Takahashi, et al, "Alignment Control for TFT-LCD" The 16th Liq. Cryst. Symp. Digest, pp. 212-213 (1990); and (4) Japanese Utility Model Publication No. 38263/1990, to shade the areas or portions, depicted with the hatchings in FIGS. 17 and 18, in which areas liquid crystal molecules are activated in response to the electric field established between the data line conductors 3 and gate bus strips 4, and the opposing electrodes 9. For instance, in the case of a twisted nematic liquid crystal display (TN-LCD), transmissivity of such areas cannot be controlled. Thus, if such areas are not shaded by the black masks, then a quality of display will be degenerated because of a lower contrast ratio.
Now, it will be described more in detail hereinafter how the liquid crystal molecules are activated in response to the irregular electric field between the data line conductors 3, gate bus strips 4, pixel electrodes 6 and the opposing electrodes 9. FIG. 19 is a view showing the distribution of lines of electric force set up between each two of the data line conductors 3, the pixel electrodes 6 and the opposing electrodes 9 in the conventional active-matrix-addressed liquid crystal display. FIG. 20 is a view showing the distribution of lines of electric force established between each two of the gate bus strips 4, the pixel electrodes 6 and the opposing electrodes 9 in the conventional active-matrix-addressed liquid crystal display.
As shown in FIG. 19, lines of electric force start from the data line conductor 3 and reach the opposing electrode 9 and the pixel electrodes 6. Particularly, a shorter distance between the data line conductor 3 and the opposing electrode 9 induces a stronger electric field therebetween, and thus transverse components of the electric field have more effects on the liquid crystal molecules. As a result, an optical leakage may be caused by irregular action of the liquid crystal molecules in the regions above the opposite, elongated sides of the data line conductors 3, and/or a disclination line may be caused by a reverse tilt on the pixel electrodes 6. This is similar also as to the matter of the gate bus strip 4 shown in FIG. 20.
According to the references (1) and (2) noted above, an electric field distribution is obtained by means of a simulation so as to locate the disclination. Further, according to the reference (3), it is obtained by an experience that such a phenomenon is involved in a tilt angle.
However, these prior art documents fail to disclose specific countermeasures against the drawbacks mentioned above. Particularly, the countermeasures disclosed in the reference (4) are disadvantageous in that a viewing angle at which the lightest view is available is apt to be varied. Further, in order to suppress the above-mentioned drawbacks even with the liquid crystal display device with a larger viewing angle available, in other words, in order to obtain a display device with a wider viewing range with high contrast in an oblique direction with respect to the display screen, the black mask 8 is required to extend over portions of the pixel electrodes 6. According to the references (1) and (2), the extending portions were required to be in the order of at least 20 .mu.m. However, the ratio of apertures to the whole area, or aperture ratio, of the black mask 8 is smaller with the higher pixel density of the liquid crystal display. Therefore, there was such a problem that although the extended portions of the black mask 8 are preferably smaller, they were not accomplished.
In addition, since the reverse tilt disclination line 11, FIG. 21, appears due to the electric field distribution set up between either two of the data line conductors 3, the gate bus strips 4, the pixel electrodes 6 and the opposing electrodes 9, invading into the effective pixel areas at a corner of the pixel electrodes 6, it extends into the aperture from the extending portions of the black mask 8, and thus it will be a cause of the degradation in a quality of display. If these areas are shaded with the black masks, the effect of the disclination can be eliminated. However, there remains such a problem that a brightness of an image displayed is lower owing to a further decreased aperture ratio of the black mask area.